1. Field of the Invention
The present invention relates to a windshield wiper assembly and a synchronous signal generating device for a windshield wiper and, more particularly, to a windshield wiper assembly including a pair of wiper blades for wiping windshield glass and a pair of motors for moving the wiper blades independently of each other to effect wiping, as well as a synchronous signal generating device which can be used for the windshield wiper.
2. Description of the Related Art
Conventionally, a windshield wiper assembly is known which includes a pair of wiper blades and a pair of motors for moving the wiper blades, respectively, independently of each other to effect wiping. With this type of windshield wiper assembly, since each wiper blade is independently moved to effect wiping without using a link mechanism, there are cases where an interference of the wiper blades occurs. As a result, in order to prevent the interference, it is necessary to synchronize the wiping motions of the wiper blades by controlling the motors. U.S. Pat. No. 4,585,980 (Japanese Patent Application Laid-Open No. 59-120551) discloses such a synchronizing method in which one wiper blade is made to undergo a continuous wiping motion by continuously operating one motor, and only the other motor is subjected to on-off control in accordance with the angular positions of the two wiper blades.
With the above-described conventional technique, however, only one motor is subjected to on-off control. In addition there is another problem in that if the pair of wiper blades are stopped after they are brought in the vicinity of their lower reversing positions, the number of on-off operations of the motor increases during the starting period of wiper operation, i.e., during the early period of wiper operation when interference is most likely to occur. Furthermore, during high-speed running, the wiper blades are subjected to wind pressure, and the load acting on the wiper blades increases when they undergo wiping motions from an upper reversing position to a lower reversing position, whereas the load acting on them decreases when they undergo a wiping motion from the lower reversing position to the upper reversing position. For this reason, if the same synchronous control is effected irrespective of the wiping direction as in the conventional manner, there is a problem in that smooth, continuous wiping motions are hampered and the operational feeling deteriorates when the position of the driver-side wiper blade and that of the passenger-side wiper blade are considerably offset from their proper positions relative to each other.
FIG. 44 illustrates a conventional synchronous signal generating device used for synchronous control This synchronous signal generating device is attached to a side surface of a worm wheel 10, and has a cam plate 12 with toothed projections formed around its outer periphery. A sliding contact spring 14 is adapted to come into contact with each toothed projection of the cam plate 12, and a grounded sliding contact spring 15 is constantly brought into contact with the cam plate 12 in the vicinity of the toothed projections. According to this synchronous signal generating device, the rotation of the worm wheel 10 allows the sliding contact springs 14 and 15 to be made electrically conductive by means of the toothed projections of the cam plate 12, thereby outputting a synchronous signal consisting of a pulse train.
With the above-described synchronous signal generating device, however, if an attempt is made to produce a synchronous signal consisting of a multiplicity of pulses so as to accurately control the position of the wiper blade, it is necessary to increase the number of toothed projections formed around the outer periphery of the cam plate 12. If the number of the toothed projections is thus increased, the width of each toothed projection and the respective intervals between adjacent the toothed projections become small. Meanwhile, contacting portions of the sliding contact springs 14 and 15 are required to have areas of predetermined sizes. Hence, there is a problem in that limitations occur with respect to the pulse width of the synchronous signal produced, thereby making it impossible to output a synchronous signal consisting of a multiplicity of pulses. In addition, the increase in the number of toothed projections leads to increased production cost.